The invention relates to a metering valve arrangement of the type suitable for use in an advance arrangement for controlling the timing of fuel delivery by a high pressure fuel pump of a compression ignition engine. In particular, the invention relates to a metering valve arrangement for use in an advance arrangement having a light load advance scheme to permit the timing of fuel delivery by the pump to be varied depending on the load under which the engine operates.
In a diesel engine of an alternator or generator set it is necessary to vary the fuelling level to the engine in response to changes in engine load so as to ensure engine operation is maintained at a substantially constant speed. Typically, a metering valve is arranged to control the supply of fuel from a transfer pump to a high pressure rotary fuel pump which delivers fuel at high pressure to the engine. The rotary fuel pump includes a cam ring which is angularly adjustable with respect to a pump housing. The cam ring includes a plurality of cam lobes and encircles part of a distributor member which includes pumping plungers which are slidable within respective bores of the distributor member to cause pressurisation of fuel within an associated pumping chamber. The pumping plungers have associated respective shoe and roller arrangements, the rollers of which are engagable with the cam surface of the cam ring.
The output pressure of the transfer pump (referred to as xe2x80x9ctransfer pressurexe2x80x9d) is controlled so as to be related to the speed of operation of the engine with which the pump is being used. Rotation of the distributor member relative to the cam ring causes the rollers to move relative to the cam ring, engagement between the rollers and the cam lobes thereby causing the plungers to be forced in a radially inward direction to pressurise fuel within the respective bore and causing fuel to be delivered by the pump at relatively high pressure. By altering the angular position of the cam ring by means of an advance arrangement, the timing at which fuel is delivered by the pump can be adjusted.
A servo-advance scheme is provided to adjust the timing of fuel delivery by the pump in response to changes in engine speed. A light load advance arrangement may also be provided, including a light load sensing piston which is movable relative to the advance piston against the action of a light load control spring. A force due to fuel pressure within a light load control chamber acts on the light load piston, in combination with the light load control spring, to determine the relative axial positions of the light load piston and the advance piston and, hence, the maximum permitted degree of advance. The light load advance scheme also adjusts the characteristics of the servo-advance.
The metering valve controlling the level of fuelling to the high pressure pump is also operable to control the pressure of fuel within the light load control chamber (signal pressure) depending on the load under which the engine is operating. The metering valve is configured such that, depending on the engine load, the pressure of fuel acting on the light load piston varies and the position of the light load piston changes. The metering valve includes a metering valve member provided with a first port which cooperates, in use, with a first outlet port through which fuel flows to low pressure (e.g. the cam box) to vary the pressure of fuel within the light load control chamber. The metering valve member is angularly adjustable within a metering valve bore provided in a housing within which the first outlet port is defined, the rate of flow of fuel to low pressure, and hence the pressure of fuel within the light load control chamber, being varied by adjusting the angular position of the metering valve member within the bore.
The metering valve is also provided with a second port which cooperates with a second outlet port in the housing to regulate the level of fuelling, and hence the pressure of fuel (filling pressure), delivered to the pumping chambers of the high pressure pump. The first and second ports in the metering valve member are positioned such that the desired relationship between signal pressure and filling pressure is achieved at a given engine speed.
Due to variations in governor components during manufacture and due to different engine and alternator requirements upon installation and wear of such components in use, the provision of a droop control arrangement is desirable in alternator sets. Droop control permits control over the change in engine speed which must occur in order for the metering valve member to move between its fully open and fully closed positions. In single speed alternator and generator sets, it is known to provide a droop adjustment arrangement which permits the axial position of the metering valve member within the metering valve bore to be varied so as to alter the relationship between the angular position of the metering valve member and the fuel flow rate through the metering valve.
The provision of a light load advance scheme in an alternator set for single speed applications is considered to be an essential requirement for emissions purposes. It is an object of the present invention to enable this to be achieved.
According According to a first aspect of the present invention there is provided a metering valve arrangement comprising;
a metering valve member which is angularly adjustable within a metering valve bore provided in a metering valve housing,
an adjustment arrangement for adjusting the axial position of the metering valve member within the metering valve bore,
a first opening provided in the metering valve member which is registerable with a first outlet provided in the metering valve housing to control a first rate of flow of fluid through the first outlet depending on the angular position of the metering valve member within the bore, and
a second opening provided in the metering valve member which is registerable with a second outlet provided in the metering valve housing to control a second rate of flow of fluid through the second outlet, wherein the first and second outlets and the first and second openings are shaped and configured to ensure the first rate of flow of fluid maintains a substantially constant relationship to the second rate of flow of fluid for any axial position of the metering valve member within the metering valve bore.
The metering valve arrangement is particularly suitable for use in an advance arrangement of the type including an advance piston which is moveable within a first bore to adjust the timing of fuel delivery by a high pressure pump and a light load advance arrangement comprising a light load piston moveable relative to the advance piston to adjust the timing of fuel delivery under light load conditions in response to a load-dependent fuel pressure within a light load control chamber, wherein the first outlet of the metering valve arrangement is arranged to communicate with a low pressure drain to control fuel pressure within the light load control chamber and the second outlet port is arranged to communicate with a high pressure pump.
If known advance arrangements of this type, the provision of a droop control arrangement is incompatible with a light load advance arrangement as any adjustment for droop of the metering valve alters the relationship between the pressure of fuel delivered to the pump (filling pressure) and the pressure of fuel within the light load advance arrangement (signal pressure). However, by using the metering valve arrangement of the present invention, any adjustment for droop does not alter the relationship between filling pressure and signal pressure as the first and second outlet ports and the first and second openings are configured to ensure a substantially fixed relationship is always maintained between the first and second fuel flow rates (i.e. a fixed relationship is maintained between filling pressure and signal pressure) for any axial position of the metering valve member within the metering valve bore.
Preferably, the first opening is positioned in relation to the second opening, and the first outlet is positioned in relation to the second outlet, such that for any axial position of the metering valve member within the bore, the first fuel flow rate maintains a substantially constant relationship to the second fuel flow rate.
In a preferred embodiment, the valve housing takes the form of a metering valve sleeve having a tubular side wall within which the first and second outlets are defined.
In a further preferred embodiment, the first outlet has first and second control edges which are substantially perpendicular to one another, and the first opening has first and second control edges which are substantially perpendicular to one another, the first and second control edges of the first outlet and the first and second control edges of the first opening together defining a first area of overlap which determines the rate of flow of fuel through the first outlet, in use.
Similarly, the second outlet has first and second control edges which are substantially perpendicular to one another, and the second opening has first and second control edges which are substantially perpendicular to one another, the first and second control edges of the second outlet and the first and second control edges of the second opening together defining a second area of overlap which determines the rate of flow of fuel through the second outlet, in use.
In a preferred embodiment, the first and second control edges of each of the first opening, the first outlet, the second opening and the second outlet are arranged such that the first area is always substantially equal to the second area, irrespective of the axial position of the metering valve member within the metering valve bore (i.e. for all operating positions of the metering valve member within its bore).
The second control edge of the first outlet and the second control edge of the second outlet are preferably arranged at substantially the same axial position along the metering valve sleeve, the second control edge of the first opening and the second control edge of the second opening are arranged at substantially the same axial position along the metering valve member, the first control edge of the first outlet and the first control edge of the second outlet are circumferentially spaced around an internal diameter of the metering valve sleeve by substantially 180 degrees and the first control edge of the first opening and the first control edge of the second opening are circumferentially spaced around an outer surface of the metering valve member by substantially 180 degrees.
In a still further preferred embodiment, each of the first opening, the second opening, the first outlet and the second outlet has an outer periphery of substantially square or rectangular form.
According to a second aspect of the present invention there is provided an advance arrangement for use in controlling timing of fuel delivery by a fuel pump, the advance arrangement comprising;
an advance piston which is moveable within a first bore to adjust the timing of fuel delivery by the pump,
a light load advance arrangement comprising a light load piston moveable relative to the advance piston to adjust the timing of fuel delivery under light load conditions in response to a load-dependent fuel pressure within a light load control chamber, and
a metering valve arrangement as herein described, and
a droop control arrangement for adjusting the axial position of the metering valve member of the metering valve arrangement within a metering valve bore.
In a preferred embodiment, the advance piston is arranged to cooperate, in use, with a cam arrangement of a fuel pump to adjust the timing of fuel delivery by the pump.
Preferably, the advance arrangement also includes a servo-control piston which is slidable within a further bore provided in the advance piston to control the pressure of fuel within the advance piston control chamber.
In a further preferred embodiment, the metering valve arrangement is operable to vary the rate of flow of fuel through a flow path between the light load control chamber and a low pressure drain, the advance arrangement further comprising an adjustable valve arrangement providing further means for varying a restriction to fuel flow through the flow path. The adjustable valve arrangement preferably includes a valve member which is axially adjustable within an additional bore to vary the restriction to fuel flow through the flow path, the variable restriction preferably being arranged in series with a further fixed restriction.
According to a third aspect of the present invention there is provided an advance arrangement for use in controlling timing of fuel delivery by a fuel pump, the advance arrangement comprising;
an advance piston which is moveable within a first bore to adjust the timing of fuel delivery by the pump,
a light load advance arrangement comprising a light load piston moveable relative to the advance piston to adjust the timing of fuel delivery under light load conditions in response to a load-dependent fuel pressure within a light load control chamber,
a metering valve arrangement which is operable to vary the rate of flow of fuel through a flow path between the light load control chamber and a low pressure drain, and an adjustable valve arrangement providing further means for varying a restriction to fuel flow through the flow path.
In a preferred embodiment, the adjustable valve arrangement includes a valve member which is axially adjustable within an additional bore to vary the restriction to fuel flow through the flow path, the variable restriction to fuel flow being arranged in series with a further fixed restriction to fuel flow within the flow path.
The provision of the adjustable valve arrangement provides a means of fine tuning the advance characteristic of the arrangement, whereby the degree of advance can be varied to give a required fuelling level at a given engine speed by adjusting the axial position of the adjustable valve member within the additional bore. The adjustable valve arrangement provides a means of compensating for wear of the metering valve arrangement during its service life and/or a means for compensating for manufacturing variations between metering valve arrangements having nominally identical specifications.
It will be appreciated that preferred and/or optional features of the first aspect of the present invention may also be incorporated in the metering valve arrangement of the advance arrangement of the second and third aspects of the present invention.